Abstract:

We used data from Cascadia Initiative and Blanco Transform Fault Zone array of ocean bottom seismometers and hydrophone to look at the seismicity and specially depth of the Central Cascadia Continental Margin events. Events were detected and located automatically with Antelope software package (Boulder Real Time Technologies). We looked at the seismicity in a 1-degree square region (44N-45N and 124W-125W) of the Cascadia Subduction Zone from 2012 to 2013. There were about 40 events in our catalog which was more than any other land network catalogs as we expected. Automatic earthquake location procedure sometimes associated with respectively large residuals for sharp arrivals for stations in strongly heterogeneous structure that associated with continental margin and subduction zones. These large residuals force us to remove them from location procedure because they cannot be associated in a simple velocity model. For smaller events that even can remove the event from the catalog because of small number of sufficient arrival in close stations. We selected 7 larger events (with local magnitudes greater than 2) which were located by other networks before to preform accurate depth analysis. We used and 2.5D model based on the position along one of ORWELL Lines (Gerdom et al., 2000) that are closed to the area (44.75N). We located hypocenters in resulting 3D model in NonLinLoc (Lomax et al., 2000). We used a very fine grid spacing of 500 m, different distance weighting and phase inclusion/omission strategies to locating hypocenters in NonlinLoc. In order to reduce the azimuthal gap, we combined the OBS data with land station data. Our
earthquake relocation results in overall shift of 7 km to the west in seismicity pattern. Based on depth results 1 out of 7 events occurred in overriding plate. 5 events seem be occurred on plate interface and one event depth is located in the base of oceanic crust. Plate inter face in our model is located in about 15 km depth. In order to get better understanding of seismicity and including more reliable S arrivals in our analysis we have to adjust Vp/Vs ratio on continental block of the model and relocate all events on continental margin with grid-search methods that contains S arrival picks as well.